dm init: add dm-mod.waitfor to wait for asynchronously probed block devices

Just calling wait_for_device_probe() is not enough to ensure that
asynchronously probed block devices are available (E.G. mmc, usb), so
add a "dm-mod.waitfor=<device1>[,..,<deviceN>]" parameter to get
dm-init to explicitly wait for specific block devices before
initializing the tables with logic similar to the rootwait logic that
was introduced with commit cc1ed7542c8c ("init: wait for
asynchronously scanned block devices").

E.G. with dm-verity on mmc using:
dm-mod.waitfor="PARTLABEL=hash-a,PARTLABEL=root-a"

[ 0.671671] device-mapper: init: waiting for all devices to be available before creating mapped devices
[ 0.671679] device-mapper: init: waiting for device PARTLABEL=hash-a ...
[ 0.710695] mmc0: new HS200 MMC card at address 0001
[ 0.711158] mmcblk0: mmc0:0001 004GA0 3.69 GiB
[ 0.715954] mmcblk0boot0: mmc0:0001 004GA0 partition 1 2.00 MiB
[ 0.722085] mmcblk0boot1: mmc0:0001 004GA0 partition 2 2.00 MiB
[ 0.728093] mmcblk0rpmb: mmc0:0001 004GA0 partition 3 512 KiB, chardev (249:0)
[ 0.738274] mmcblk0: p1 p2 p3 p4 p5 p6 p7
[ 0.751282] device-mapper: init: waiting for device PARTLABEL=root-a ...
[ 0.751306] device-mapper: init: all devices available
[ 0.751683] device-mapper: verity: sha256 using implementation "sha256-generic"
[ 0.759344] device-mapper: ioctl: dm-0 (vroot) is ready
[ 0.766540] VFS: Mounted root (squashfs filesystem) readonly on device 254:0.

Signed-off-by: Peter Korsgaard <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/admin-guide/device-mapper/dm-init.rst

···
     0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
     fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
     51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
+
+For setups using device-mapper on top of asynchronously probed block
+devices (MMC, USB, ..), it may be necessary to tell dm-init to
+explicitly wait for them to become available before setting up the
+device-mapper tables. This can be done with the "dm-mod.waitfor="
+module parameter, which takes a list of devices to wait for::
+
+  dm-mod.waitfor=<device1>[,..,<deviceN>]

drivers/md/dm-init.c

···
  */
 
 #include <linux/ctype.h>
+#include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/device-mapper.h>
 #include <linux/init.h>
···
 #define DM_MAX_DEVICES 256
 #define DM_MAX_TARGETS 256
 #define DM_MAX_STR_SIZE 4096
+#define DM_MAX_WAITFOR 256
 
 static char *create;
+
+static char *waitfor[DM_MAX_WAITFOR];
 
 /*
  * Format: dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
  * Table format: <start_sector> <num_sectors> <target_type> <target_args>
+ * Block devices to wait for to become available before setting up tables:
+ * dm-mod.waitfor=<device1>[,..,<deviceN>]
  *
  * See Documentation/admin-guide/device-mapper/dm-init.rst for dm-mod.create="..." format
  * details.
···
 	struct dm_device *dev;
 	LIST_HEAD(devices);
 	char *str;
-	int r;
+	int i, r;
 
 	if (!create)
 		return 0;
···
 	DMINFO("waiting for all devices to be available before creating mapped devices");
 	wait_for_device_probe();
 
+	for (i = 0; i < ARRAY_SIZE(waitfor); i++) {
+		if (waitfor[i]) {
+			DMINFO("waiting for device %s ...", waitfor[i]);
+			while (!dm_get_dev_t(waitfor[i]))
+				msleep(5);
+		}
+	}
+
+	if (waitfor[0])
+		DMINFO("all devices available");
+
 	list_for_each_entry(dev, &devices, list) {
 		if (dm_early_create(&dev->dmi, dev->table,
 				    dev->target_args_array))
···
 
 module_param(create, charp, 0);
 MODULE_PARM_DESC(create, "Create a mapped device in early boot");
+
+module_param_array(waitfor, charp, NULL, 0);
+MODULE_PARM_DESC(waitfor, "Devices to wait for before setting up tables");